1. Field of the Invention
The invention relates to a photo-imprinting stand for the making of Bragg gratings.
It can be applied in the different methods used for the photo-imprinting of Bragg gratings on all types of photosensitive optical waveguides and especially on optical fibers.
Hereinafter, the terms `photosensitive optical waveguide` and `optical fiber` shall be used without distinction, it being clear that a fiber section is nothing other than an optical guide.
2. Description of the Prior Art
It may be recalled that a photo-imprinting stand for the making of Bragg gratings comprises, in a known way, a radiation source 1 capable of insolating an optical waveguide or optical fiber 4. The insolation is done by means of an optical device 2 used to obtain an interference field.
A schematic drawing of a photo-imprinting stand is shown in FIG. 1.
The photo-imprinting of a Bragg grating consists of the insolating of a fiber by means of a system of ultraviolet interference fringes in order to create a permanent variation in index at the core of the fiber that is periodic along its axis. The index modulation depth is a function of the power received as well as the exposure time.
The interference fringes can be obtained by different methods, namely the use of a phase mask or a method of inteferometry.
The recording can be done either through a scanning by an ultraviolet beam before the guide or by means of an extended beam, namely a beam that irradiates the entire guide.
In FIG. 1, it has been assumed that a ultraviolet beam with a small width is used and the shifting of this beam is symbolized by the arrow D shown in dashes.
The reference 3 symbolizes the interference field obtained by the optical system 2.
In recent years, the making of Bragg gratings has undergone development.
Indeed, these gratings used in reflection or in transmission are introduced into many optical functions performed by devices such as lasers, demultiplexers, chromatic dispersion compensators, sensors, etc.
Their uses in certain applications have revealed certain requirements as regards their characteristics. A standard Bragg grating has a reflection spectrum that possesses substantial smearing, namely residual reflection outside the band. Furthermore, the reflection in the neighborhood of the Bragg wavelength is not constant. The chromatic dispersion compensator gratings must possess a linear dispersion. This set of constraints requires that the gratings should be apodized, namely that it is necessary to create a continuous variation of the index modulation depth at the ends of the grating. Various methods of apodization have already been proposed.
In a first approach, for example, the interference fringes at the ends of the grating are scrambled. Reference can be made for further details to the document [1] "Simple Technique for Apodizing Chirped and Unchirped Fiber Bragg Gratings", in Electronics Letters, 20th June 1996, Vol. 32, No. 13 or the document [2] entitled "Moving Fiber/Phase Mask-Scanning Beam Technique For Enhanced Flexibility in Producing Fiber Gratings With Uniform Phase Mask" in Electronics Letters, 17th August 1995, Vol. 31, No. 17.
A second approach consists of the use of a specific phase mask having a variable diffraction efficiency. Reference can be made for further details to the document [3] entitled "Apodization of the Spectral Response of Fiber Bragg Gratings using a Phase Mask with Variable Diffraction Efficiency" published in Electronics Letters, 2nd February 1995, Vol. 31, No. 3.
A third approach consists in obtaining a variation in the intensity by using a wide beam having an appropriate profile,: for example a Gaussian profile, or by using beam masking operations and carrying out for example a twofold exposure of the guide. The first exposure is done in the presence of a mask and without a phase mask and the second exposure is done in the presence of the phase mask and a second mask forming a "bell-shaped" beam profile that is complementary to the mask of the first exposure. This twofold exposure gives a mean index elevation that is constant along the grating. Reference can be made for further details to the document [4] entitled "Apodized In-Fiber Bragg Grating Reflectors Photo-Imprinted Using A Phase Mask" published in Electronics Letters, 2nd February 1995, Vol. 31, No. 3.
This last-named technique is cumbersome to implement. Moreover, present-day techniques of power variation along the grating by masking or by the use of a wide beam with an adapted profile do not permit high reproducibility of the ultraviolet power profile along the fiber.
The techniques using a scrambling of fringes at the ends of the grating during the photo-imprinting cannot be used for strict control over the profile of the desired depth of modulation. Furthermore, not every type of profile can be obtained.
Since phase masks with a variable diffraction efficiency can be used only for one model of grating, each model requires a different mask.